Workflow tabs show a number of tasks which don't exist

Question

Workflow tabs show a number of tasks which don't exist. How can I clean this counter?

Answer

The problems were "ghost" workflows which were started and already finished (with unexpected errors, or in case of a migration). 

To clean these errors, you have to go to Jahia tools --> Workflow monitoring

In this workflow monitoring tool, you can see all open tasks. For the ghosts tasks, when you click on the MainNodeId it should lead to a non-existent JCR Node. In this case, you can remove the workflow with the "abort workflow" link. This should stop the open tasks without a workflow.